The present invention relates to a method and system for controlling spark ignition of a fuel/air mixture contained in the combustion chamber of an Otto engine, of the type including an ignition voltage-generating ignition system comprising at least one spark plug electrode for spark ignition fixedly arranged in each combustion chamber and with an earth electrode co-operating movably therewith and joined fixedly to the engine piston in question.
Two-part ignition arrangements of the type mentioned above are known in a large number of embodiments, and disclosed for example, by U.S. Pat. Nos. 1,623,432 and 2,253,204. In the known solutions, as a result of the division of the ignition arrangement into two parts, simple ignition systems are used for generating the ignition voltage to the ignition arrangement in question. However, the moment of the ignition spark formation occurs in an uncontrolled manner by virtue of the fact that ignition takes place when the electrodes are sufficiently close to one another to cause spark formation under the prevailing pressure and temperature conditions. Solutions of this type may be adequate for engine applications where demands on fuel consumption, exhaust gas emissions and performance are comparatively small and/or where the engine runs with small variations in the operating conditions. However, the demands placed on modern engines for operating passenger cars cannot be satisfied by solutions of this type.
An object of the present invention is to make a method and system, when using two-part ignition arrangements on modern Otto engines for motor vehicle operation, for controlling the ignition such that a well-controlled combustion is achieved under widely differing operating conditions. In this connection the invention involves the ignition moment being controlled in relation to the sparking distance such that an ignition spark necessary for satisfactory combustion of the fuel/air mixture is obtained independently of whether the engine is running at low or high load. For this purpose the invention is characterized in that, for each engine speed, ignition voltage is generated for spark formation at a first ignition moment, which at a first engine load value corresponds to a first sparking distance, while at a second engine load value, which is higher than the first, ignition voltage is generated at a second ignition moment which corresponds to a second sparking distance shorter than the first. Further, the ignition moment is controlled over essentially the whole load range for the engine in such a way that the ignition voltage level for spark formation, which in a manner known per se depends on the engine load and the sparking distance, varies within relatively narrow limits.
The invention makes possible an advantageous application of two-part ignition arrangements in modern Otto engines which run under considerably varying loads and with high demands placed on performance, fuel consumption and exhaust gas emissions, such as is the case in operation of cars. For the foregoing type of engines, the solution according to the invention ensures, at each engine speed, spark formation over a predetermined long sparking distance when the engine is running at a low load and over a successively shorter sparking distance as the load increases. The ignition voltage and with it the spark energy can thus be maintained at a high level over the whole load range, which ensures satisfactory ignition and combustion of the fuel/air mixture within the whole range mentioned. The risk of incomplete combustion with, as a consequence, increased exhaust gas emissions and impaired exhaust gas catalyst function is thus reduced, and at the same time the engine runs economically in terms of fuel and achieves smooth running which is desirable from the point of view of comfort.
In an advantageous embodiment of the invention the ignition moment is controlled such that the ignition voltage essentially varies by less than 20 percent about a mean value defined by the limits. This means in practice that the above-mentioned ignition voltage level, and the spark energy which is essentially in proportion thereto in every respect, is considerably raised particularly at low engine load compared with the levels which occur in known engines, the ignition arrangements of which have a fixed sparking distance. In this way the risks of unsatisfactory ignition of the fuel/air mixture at low engine load can be considerably reduced.